Cyclomaltodextrin glucanosyltransferase

A. oryzae a-amylase consists of three domains (A, B and C).179,180 Domain A has an amino-terminal ((3/a)8-barrel structure, followed by domain C, consisting of (3-strands that are folded into a Greek motif. Domain B is inserted between the third (3-strand and the third a-helix of the ((3/a)8-barrel. This is a highly variable domain in both its length and amino acid sequence, depending on the source and type of the enzyme.181,182 Cyclomaltodextrin glucanosyltransferases generally consist of five domains (A, B, C, D and E). Domains A, B and C consist of the same catalytic domains found in... 

A few chimeric enzymes have been constructed by adding one or more domains from one amylolytic enzyme to another. Some of these chimeric enzymes have been studied in regard to the secretion of the enzyme,187 substrate specificity188 and product specificity.189 A starch binding domain from a Bacillus sp. cyclomaltodextrin glucanosyltransferase was fused with B. subtilis a-amylase, and an a-amylase was... 

An identical mechanism can be postulated for hydrolysis of the a-(l—>6) branch linkage by isoamylases and for cyclomaltodextrin glucanosyltransferase. For the latter enzyme, the water molecule is replaced by the C-4 hydroxyl group on the nonreducing end glucosyl unit of the starch chain (Figure 7.6). 

Figure 7.17 Structures of active-site directed amylase inhibitors. The K values are for glucoamylase (GA), porcine pancreatic oamylase (PPA) and cyclomaltodextrin glucanosyltransferase (CGTase).

Cycloinulohexaose was obtained by the action of an enzyme from Bacillus circu-lans OKUMZ 3IB on inulin [59]. Small amounts of cycloinuloheptaose and cy-cloinulooctaose were also obtained. The linkages in these cyclic dextrins are P-2 1, as is found in inulin. The formation of these cyclic dextrins by this enzyme is most probably similar to that of cyclomaltodextrin glucanosyltransferase and cycloisomaltodextrin glucanosyltransferase. The structure for cycloinulohexaose is given in Fig. 8.8. 

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